The radiation detector is an elemental technology that supports the technology utilizing radiation. With the recent development of the technology utilizing radiation, radiation detectors with higher performance are demanded. In the technology utilizing radiation, advancement in the technology utilizing neutrons is remarkable. Application of the technology utilizing neutrons is expanding in such fields as the academic research field including structural analysis by neutron diffraction, the field of non-destructive inspection, the security field including cargo inspection, and the medical field including boron neutron capture therapy; and accordingly, neutron detectors with higher performance are demanded.
An important characteristic demanded of the neutron detector is discrimination ability between neutrons and gamma rays (also referred to as “n/γ discrimination”, hereinafter). Gamma rays not only exist in natural radiation, but also are generated when neutrons hit a component of a neutron detecting system or hit an object to be inspected. Therefore, if gamma rays are mistakenly detected as neutrons because of low n/γ discrimination, signals not reflecting the interaction between neutrons and the object to be inspected will be included, causing increase in the so-called background noise.
In general, a neutron capture reaction is utilized to detect neutrons since neutrons have strong tendency to pass through a material without performing any interactions in the material. For example, a helium-3 detector is known, which performs the detection by utilizing a proton and tritium generated by the neutron capture reaction between 3He and neutron. This detector is a proportional counter filled with 3He gas, having high detection efficiency and being excellent in n/γ discrimination. However, there is a drawback that the size of the detector is hard to reduce. Further, 3He is an expensive substance and is also limited in its amount.
Recently, a scintillation neutron detector having a neutron scintillator has been developed as an alternative to the helium-3 detector described above. The neutron scintillator is a substance that emits light by interaction with neutrons irradiated thereto. Combining the neutron scintillator and a photodetector such as a photomultiplier tube can form the scintillation neutron detector. As the neutron scintillator, a material containing a large amount of isotope that exhibits high efficiency in the neutron capture reaction, such as 6Li and 10B, is employed in order to improve the detection efficiency for neutrons (see Patent Document 1, for example).
The scintillation neutron detector having the neutron scintillator is advantageous in that it has high detection efficiency for neutrons and can be easily reduced in size. However, it has a drawback of being poor in n/γ discrimination since the neutron scintillator is sensitive to gamma rays as well.
Various attempts have been made in view of the above problems. In the scintillation neutron detector having the neutron scintillator, a photodetector detects light emitted from the neutron scintillator and outputs a pulse signal. There has been suggested a method of discriminating between neutrons and gamma rays by using a difference in the shape of the pulse signal between neutron incidents and gamma ray incidents (hereinafter the method being referred to as pulse shape discrimination) (see Non-Patent Documents 1 and 2). A detector adopting the pulse shape discrimination can be used as a radiation detector that can measure both neutrons and gamma rays with discrimination between them.